This invention relates to a method of video signal coding.
The PAL composite video coding standard has a number of limitations that restrict the picture quality that such a signal can convey. These limitations are well known; they include limited luminance and chrominance resolution, cross-color and cross-luminance.
Several methods are known that aim at improving the image quality that can be conveyed by a PAL signal. For example, multidimensional filtering techniques may be employed to help to separate chrominance and luminance information, as described in BBC Research Department Report No. 1975/36, `The filtering of luminance and chrominance to avoid cross-color in a PAL color system`. This approach reduces significantly the effects of cross-color and cross-luminance but cannot eliminate them completely; it also forces the use of luminance and chrominance passbands that have a sub-optimum shape.
Another known technique for improving the quality of a PAL signal is the so-called phase-segregated or `Weston` clean PAL, as described in BBC Research Department Report No. 1980/1, `Weston clean PAL`; see also U.K. Patents 1,534,268-270. This coding method separates luminance and chrominance information by phase rather than frequency and thus allows perfect separation of the two signals, eliminating cross effects. It also allows a significant amount of flexibility in choosing the shapes of the passbands of the luminance and chrominance signals. In particular, the luminance part of the signal can carry samples at a rate of twice the color subcarrier frequency with negligible crosstalk between samples, allowing so-called spectrum-folding techniques to be used. However, the overall spectral volume of the luminance signal is limited to that equivalent to a signal having a bandwidth equal to the color subcarrier frequency. The resolution of the chrominance signal may however be increased by increasing the overall bandwidth of the composite signal.
A third technique has been proposed for use in situations where the bandwidth of the composite signal may be increased above the usual figure of about 5.5 MHz. (The numerical values in this specification are given by way of example and are based on the PAL System I television standard.) This technique, known as Extended PAL, is described in BBC Research Department Report No. 1981/11, An Extended PAL system for high-quality television`; see also U.K. Patent Specification 2,101,835. In this technique, an additional signal carrying high frequency luminance information is placed above the frequency band that carries the chrominance information and above the subcarrier frequency used to carry the sound signal. Luminance signals having a horizontal bandwidth above about 3.5 MHz are separated from the normal low-frequency luminance signal and frequency-shifted by the subcarrier frequency before transmission. Cross effects are virtually eliminated since there is very little overlap between chrominance and luminance signals. The horizontal resolution of the luminance signal transmissible by such a method is determined by the bandwidth of the channel available, being equal to the channel bandwidth less the color subcarrier frequency. Although the system offers an increased luminance bandwidth compared to that of other PAL coding methods, it does not have the flexibility in determining the shape of the luminance passband afforded by Weston clean PAL. It also makes sub-optimum use of the available bandwidth, since gaps need to be left between the various parts of the signal spectrum.
A fourth technique known as enhanced or compensated Weston clean PAL is described in our UK Patent Specification 2,115,638. In this technique, a "compensation signal" is formed consisting of those parts of the luminance and chrominance signal spectrum that a Weston clean PAL signal cannot represent. The compensation signal consists of high vertical frequency luminance information having a horizontal frequency between 3.3 MHz and 5.5 MHz, high vertical frequency chrominance information having a horizontal bandwidth from 0 to 1.1 MHz, and all chrominance information having a horizontal bandwidth between 1.1 and 2.2 MHz. The luminance and chrominance compensation signals are combined into one signal using a second Weston PAL assembler, and the combined signal is modulated onto a carrier at twice the subcarrier frequency. The compensation signal is separated from the normal Weston PAL signal at the decoder using simple low- and high-pass filters. This technique is limited to providing horizontal resolution up to 5.5 MHz and cannot make use of the flexibility of a basic Weston PAL coding system to allow the shape of the luminance pass-band to be optimised using spectrum folding. Further, it requires the use of sharp-cut filters to separate the normal and compensation signals. Sharp cut band-pass filters must be used in both the coder and decoder in order for the normal and compensated signals to be combined to yield a perfect reconstruction of the original signal.